Adding Header to every existing page?

and it has to be the Layerslider? Would an Enfold Slider ( full-width f.e. ) also be possible?
don’t know why layerslider shortcode does not work with this method?

for example a full-width slider and a following color-section.
style a page as you like to have the content.
copy that enfold shortcode and insert it in a hook via do_shortcode

add_action('ava_after_main_title', function() {
if (is_page(2604)){
    echo do_shortcode("[av_slideshow_full size='featured' min_height='0px' stretch='' control_layout='av-control-default' src='' attachment='' attachment_size='' position='top left' repeat='no-repeat' attach='scroll' conditional_play='' animation='slide' autoplay='true' interval='5' id='' custom_class='' av_uid='av-k85x1xh3']
[av_slide_full id='35860'][/av_slide_full]
[av_slide_full id='35865'][/av_slide_full]
[av_slide_full id='35862'][/av_slide_full]
[/av_slideshow_full]
[av_section min_height='25' min_height_pc='25' min_height_px='500px' padding='default' custom_margin='0px' custom_margin_sync='true' color='main_color' background='bg_color' custom_bg='#dddddd' background_gradient_color1='' background_gradient_color2='' background_gradient_direction='vertical' src='' attachment='' attachment_size='' attach='scroll' position='top left' repeat='no-repeat' video='' video_ratio='16:9' overlay_opacity='0.5' overlay_color='' overlay_pattern='' overlay_custom_pattern='' shadow='no-border-styling' bottom_border='no-border-styling' bottom_border_diagonal_color='#333333' bottom_border_diagonal_direction='' bottom_border_style='' custom_arrow_bg='' id='' custom_class='' aria_label='' av_element_hidden_in_editor='0' av_uid='av-k85r9uui']
[av_heading heading='".get_the_title()."' tag='h2' style='blockquote modern-quote modern-centered' subheading_active='subheading_below' size='' av-medium-font-size-title='' av-small-font-size-title='' av-mini-font-size-title='' subheading_size='' av-medium-font-size='' av-small-font-size='' av-mini-font-size='' color='' custom_font='' margin='' margin_sync='true' padding='10' link='' link_target='' id='' custom_class='' av_uid='av-k85rator' admin_preview_bg='']
Wordpress Tag Line
[/av_heading]
[/av_section]"); 
}
});

see here the result: https://webers-testseite.de/kontakt/
the pagetitle is inserted dynamically by that break in the code: ".get_the_title()."
You see that even the first color-section on that page gets the right index as #av_section_2

the basic is:

add_action('ava_after_main_title', function() {
    echo do_shortcode("[the-copied-shortcode-from-debug-mode]"); 
});

the if clause can handle all you like f.e.
if( is_single()||is_singular( 'event' )||is_search()||is_category()||is_page()|| is_singular( 'portfolio' )){

OK :
The first thing I would always advise on WordPress is to use a child theme.
Link for a predefined Child-Theme and some instructions: https://kriesi.at/documentation/enfold/child-theme/

The advantages of a child theme usage greatly outweigh the few disadvantages; ; last but not least, the desired changes will be preserved with the next update.

The downloaded Child-Theme of Enfold has three files – one png is only to have a preview image in the themes section of your wordpress dashboard.
The child theme style.css is absolutely necessary to identify itself as a child of Enfold. And the function.php which is at the beginning empty ( only some outcommented lines and the starting code.
This child-theme functions.php can be used now to have additional code to the parent theme.

One of this could be to activate the debug mode of Enfold:
So this comes to child-theme functions.php:

// Debug Mode of Enfold
function builder_set_debug(){
  return "debug";
}
add_action('avia_builder_mode', "builder_set_debug");

The first line is only to remember what we like to influence – on php code the two slashes implicates a one line comment.
From now on you have on every Enfold styled Page a Field under the layout field in Editor Mode: See here a simple Setting with one full-width slider and a color-section under it – with some heading:
(click to enlarge)

Now you can see below the layout field how Enfold works “under the hood”.
these shortcodes can be copied and used on different places. Copy the whole shortcode and replace: [the-copied-shortcode-from-debug-mode]

add_action('ava_after_main_title', function() {
    echo do_shortcode("[the-copied-shortcode-from-debug-mode]"); 
});

the resulting code then comes to child-theme functions.php.
It might be good for posts and portfolio pages with sidebar not to have on the second place of the designing auxiliary site to have a color-section : because a color-section is a fullwidth element as the slider – so sidebar will start under this.
So for your needs it would be better to have a full-width slider first and then a 1/1 container for the Headings under it.

that is a very nice solution!
– but with it it is not possible to have dynamic content like the “get_the_title()” or something like this in it ?
btw ( for me it works nice on every page without the if clause )

In your Screenshot first post – you like to only have the bloginfo name and description for that? I thought you like to have the post title on it.

in this case we can use on that page a shortcode to insert dynamic content like the page title.
this would generate a page title shortcode which we can use

function post_title_shortcode(){
    return get_the_title();
}
add_shortcode('post_title','post_title_shortcode');

you then can use [post_title] to have the page-title in your “recurring page content”